The present invention relates to a method of recovering cis-1,4-dichlorobutene-2 (1,4-DCB-2) in high purity. More particularly, the present invention relates to a distillation process.
Cis-1,4-dichlorobutene-2 is one of the products resulting from the addition of chlorine to butadiene, for example, the process of chlorinating butadiene to give a mixture of 3,4-dichlorobutene-1 and cis, trans-1,4-dichlorobutene-2, isomerizing the latter to 3,4-dichlorobutene-1 and dehydrochlorinating the 3,4-dichlorobutene-1 to chloroprene is well known. All three isomers are in equilibrium with one another so that it is theoretically possible to convert the entire dichlorobutene mixture into any one isomer, however in a commercial size operation there still may be a substantial quantity of by-product.
Previously cis-1,4-dichlorobutene-2 was a by-product which was desirably converted to the chloroprene precursor, however, recently the cis-1,4-DCB-2 has been found to be useful as a precursor for cis-1,4-dichloro-2,3-epoxybutane, which has lead to a new class of polyethers (U.S. Pat. No. 3,065,188).
The cis and trans-1,4-dichloro-2,3-epoxybutanes useful in the polymerizations may be prepared by the peracetic acid epoxidation of the respective dichloro-2-butenes. Such a process is described in U.S. Pat. No. 3,150,154. Cis-reaction produces the cis-oxide and trans-reactant produces the trans-oxide.
Recent studies as reported by E. J. Vandenberg in a paper entitled "A New Class of Polyethers - Poly (1,4-Dichloro-2,3-Epoxybutane)S - Synthesis, Mechanism and Property Aspects" presented at the IUPAC Microsymposium on "Polymerization of Heterocycles", Jablonna, Poland, June 23-25, 1975 established that the cis form of the oxide produced the most valuable polymers.
The separation of the cis-1,4-dichlorobutene-2 may be carried out from a racemic mixture of cis- and trans-1,4-dichlorobutene-2 and 3,4-dichlorobutene-1. The separation of the 3,4-dichlorobutene-1 from the other isomers can be effected to a high degree by straight distillation, there is, however a small amount of isomerization (principally a thermal isomerization even under reduced pressure conditions) of the remaining cis- and trans-1,4-DCB-2 so that after the cis- and trans-isomers have been separated by distillation a subsequent distillation may be necessary to obtain high purity cis-1,4-DCB-2.
It is a feature of the present invention that an improved vaporization ratio of cis to trans-1,4-dichlorobutene-2 is obtained in distillation to separate the cis and trans-isomers. It is a further feature of the present invention that a distillation process for recovering high purity cis-1,4-dichlorobutene-2, trans-1,4-dichlorobutene-2 and 3,4-dichlorobutene-1 is provided.